The present invention relates to the production of information display panels having at least one zinc sulfide powder electroluminescent layer, in which the electroluminescent (EL) powder is coactivated with aluminum and is formed as a monoparticle layer embedded in a high dielectric constant thermoplastic resin which is protected against voltage breakdown by thin evaporated oxide layers on both sides. The resin is preferably partially dyed with organic light-absorbing dyes to improve the contrast and/or fluorescent dyes for the production of white or colored light.
Such display panels are useful for color television screens as well as numerical displays or other types of information display panels. The EL powder layer embedded in high dielectric constant resin, together with a transparent front electrode and a black rear electrode, forms a capacitive light-emitting element which is excited into emission by application of a high-frequency alternating voltage. A plurality of such capacitive light-emitting elements can be arranged in a panel to form, for example, a seven-segment display or a TV matrix.
The use of an EL powder layer results in a very small layer thickness corresponding to the diameter of a single powder grain. This is less than the thickness of known single-crystal layers such as are disclosed in this relation in Motson U.S. Pat. No. 3,037,138. The use of phosphors in powder form is known from Lehmann U.S. Pat. No. 2,924,732, although with no suggestion of a monoparticle layer. Lehmann also teaches the following:
(a) The so-called "cascade electroluminescence", that is, the mixture of an EL powder as a primary emitter with a photofluorescent material. Thus, to obtain a red component, a short-wave blue-emitting EL material is mixed with one or two components having longer wave red fluorescence. The basis for this practice lies in the generally known characteristic of EL powder layers that they can emit blue, green or yellow-orange light when used with phosphors such as those of Lehmann, but white light can be obtained only by use of zinc sulfide powder mixtures. Therein lies a further essential difference of the last-mentioned powder layers from vapor-deposited EL layers which emit yellow light. With cascade luminescence, white light can be obtained from blue-luminescing EL powder by admixture with green, yellow or red fluorescing organic materials, and with no color change in aging.
(b) The admixture of organic, preferably red fluorescing pigments, for example, rhodamine B.
(c) Coactivation with aluminum in order to make the emitted color value largely independent of the exciting frequency.
A difficult problem for practical use (in color TV) is presented by aging of the EL layers which results in loss of light intensity of the individual components with resulting change in color value or hue.